1. Field of the Invention
The present invention relates to a semiconductor device having a plurality of GaAs metal-semiconductor field effect transistors (to be referred to as "GaAs MES FETs" hereinafter), and more particularly, to an improvement of frequency characteristic of a semiconductor device, such as a GaAs MES FET, in an ultra-high frequency band, such as a microwave frequency band.
2. Description of Related Art
FIG. 1 is a diagram showing a 50 .OMEGA. impedance-matching circuit of a semiconductor device for power amplification which is used in an ultra-high frequency band, such as a microwave frequency band. Referring to FIG. 1, the semiconductor device includes two GaAs MES FET chips ("FET chips" hereinafter) 101 on which two FET circuits are respectively formed. Each FET circuit has 4 GaAs MES FET elements. The FET circuits are connected to each other by two bonding wires 109. The FET chips 101 are mounted on a package (not shown). Capacitor chips 102 are respectively provided on both sides of the FET chips 101, i.e., the input and output sides of the FET chips. Each capacitor chip 102 is provided with 2 chip capacitors 102-1 and 102-2. As shown in FIG. 2, each chip capacitor is formed of a dielectric substance 111 having a height H and a relative dielectric constant .epsilon..sub.r, an upper surface metalized pattern 105 as an upper electrode having a width W and a length L, and a lower electrode 106. Input or output terminals of each FET chip are connected to the corresponding chip capacitor by 4 bonding wires 108 in parallel. Dielectric substance substrates 103 are respectively provided on the outsides of the capacitor chips 102, i.e., on the input and output sides. A microstrip transmission line 104 having branch ports in the shape of ring with a small gap and open stub patterns 107 is formed on each of the substrates 103. Each microstrip transmission line 104 is connected each of the corresponding chip capacitors by 4 bonding wires. In this manner, on the semiconductor circuit, a concentrated parameter circuit is constituted of the chip capacitors 102-1 and 102-2, bonding wires 108 and the like, and a distributed parameter circuit is constituted of the microstrip transmission line 104 and the open stub patterns 107 formed on the dielectric substance substrate 103.
The upper surface metalized pattern is conventionally formed for each FET circuit. The frequency characteristic of the semiconductor device in this case is shown in FIG. 3. The frequency characteristic has a non-flat portion within a practical frequency band (.+-.5 to 15%) having a center frequency of 14.25 GHz, for example, and has a local portion with gain greatly reduced, i.e., a discontinuous point, as shown in FIG. 3.
There are problems with the semiconductor circuit operating in an ultra-high frequency band in the following matters.
That is, the upper surface metalized pattern having a length L which is formed on the surface of a capacitor chip has a resonance frequency within an operation frequency band. Therefore, there is caused a problem in that the operation of the semiconductor device becomes unstable and a discontinuous point of gain, i.e., a point at which gain is locally reduced appears within the operation frequency band.
The FET chips or GaAs FET elements are manufactured in the same manner but are not completely identical so that a very small unbalance exists among the GaAs FET elements. As a result, a signal which is amplified using the bonding wires, the upper surface metalized pattern on the chip capacitor, and the dielectric substance substrate as medium is transferred between the FET circuits or GaAs FET elements from one to the other. Such a phenomenon adversely influences the matching state of the semiconductor device and, therefore, the operation of semiconductor device becomes unstable such that the discontinuous point appears.
In a case where the capacitor chips are fixed to the package with solder such as AuSn, bubbles are sometimes taken into the AuSn solder, partially producing portions where the lower surface metalized pattern of the capacitor chip does not contact the ground portion of the package, resulting in a random distribution of capacitance. For this reason, a relative potential difference is induced between the upper surface metalized patterns on the capacitor chip, or between the branch ports on the dielectric substance substrate. This also causes the discontinuous point within the operation frequency band.